Maybe the "windows" is not the right word, but rather the little doors on the Advent calendar.

You can see one piece of the chocolate on the fifth photo.

It is below the door 2 (on the door 13).

These chocolates contain 33% cocoa.

If you zoom in on the photo, then the chocolate is visible.

Probably most advent calendars are similar.

But maybe not all of them has milk chocolate.

Have a pleasant Advent.